1. Field of the Invention
The present invention relates to a sensor control apparatus and a sensor control method, each of which is suitable for application to a sensor used for controlling an internal combustion engine, in particular, a gas sensor for measuring a concentration of the particular gas component.
2. Description of the Related Art
In general, in an internal combustion engine used as the driving source of a movable body such as a vehicle, a control for optimizing a combustion parameter in the engine has come into wide use in order to improve an output power or a fuel cost or purify exhaust gas, for example. As the combustion parameter, there is an air-fuel ratio which is obtained by dividing an air mass in a mixture of the air and fuel burnt in the engine by a fuel mass. The air-fuel ratio is controlled so as to be a value suitable for an operation state of the engine, for example, a theoretical air-fuel ratio for combusting the air and fuel in just proportion or a predetermined value near the theoretical air-fuel ratio.
In the case of actually performing the aforesaid control, an accurate estimate of the air mass in the air-fuel ratio, more concretely, the oxygen mass is needed. Accordingly, a technique has been developed in which an air flow sensor for measuring the flow rate of the air sucked into the engine and an oxygen sensor for measuring the concentration of oxygen gas remaining in burned exhaust gas, for example, are provided to more accurately obtain an air-fuel ratio (see, for example, Patent Literature 1).
In the aforesaid oxygen sensor, in general, the measurement accuracy of oxygen concentration depends on the temperature of a sensor element as a measuring part. Thus, the oxygen sensor is provided with a heater, etc., for controlling the temperature of the sensor element in a predetermined range. Further, in a related art oxygen sensor, a base part, protruding outside of a flow path of the exhaust gas at the time of being attached to the engine, is configured by assembling plural members each formed by processing a thin metal plate. However, according to the method of configuring the base part using the thin metal plate, problems have been encountered in that the structure becomes complicated and the assembly procedure is troublesome.
In view of the above, a technique has been proposed in which the base part of the oxygen sensor is configured by a resin member which can be processed easier than the metal thin plate (see Patent Literature 2). Since the base part is configured by the resin member capable of forming a complicated shape, the number of components constituting the base part can be reduced, whereby the structure can be simplified and the procedure for assembly is unnecessary.    [Patent Literature 1] JP-A-10-132779    [Patent Literature 2] JP-A-2003-148206
3. Problems to be Solved by the Invention
On the other hand, in recent years, in order to reduce an amount of carbon dioxide exhausted from an internal combustion engine or to reduce fuel cost, an idling stop control has come into use. In the idling stop control, the engine is automatically stopped when a predetermined condition is satisfied such that a vehicle, etc., is stopped, and thereafter the engine is automatically started again when a driver operates to start the vehicle, etc. When the engine is automatically stopped (idling stop) by the idling stop control, since the suction of air and exhaust of the exhaust gas in the engine is also stopped, the flow of gas around the oxygen sensor also stops.
As a result, an amount of heat drawn into the ambient gas from the sensor element and a metal housing, etc., is reduced. Then, when the temperature of the sensor element heated by the heater so as to be kept at an activating temperature is maintained, the temperature of the main body of the oxygen sensor such as the housing for housing the sensor element is apt to increase. Like the oxygen sensor described in Patent Literature 2, in a sensor in which a portion above a housing is configured by a resin member having lower degree of thermal resistance than that of a metal material, a problem arises that, in the idling stop state, the resin member constituting the oxygen sensor is influenced by heat transmitted from the sensor element and hence the temperature of the resin member may exceed its heat proof temperature.